1. Field of the Invention
The invention is related to N-polar aluminum gallium nitride (AlGaN)/gallium nitride (GaN) enhancement mode field effect transistors (FETs), such as High Electron Mobility Transistors (HEMTs).
2. Description of the Related Art
Group III-nitride based high electron mobility transistors (HEMTs) are attracting significant interest for power switching applications owing to the possibility of delivering high breakdown voltages (VBD) and low on-resistance (RON) beyond the material limits of Si and SiC.
Enhancement mode (E-mode) or normally-off devices based on GaN technology are interesting for a variety of applications, and are especially desirable for power switching applications due to the added safety of a normally off device.
In devices grown in the Ga-polar or (0001) direction, E-mode operation is achieved in AlGaN/GaN buffer structures by etching away some of the AlGaN under the gate region (method-1), exposing the AlGaN under the gate region with a fluorine-based plasma (method-2), or by capping the AlGaN layer with a p-type GaN under the gate (method-3).
Threshold uniformity is especially important in large periphery high breakdown devices. Since AlGaN (and GaN) are most easily etched via dry etching (method-1), achieving uniform threshold voltage and repeatability is extremely difficult, and thus this method will remain unattractive without an etch-stop layer. Fluorine-plasma treatment (method-2) achieves threshold-voltage shift by a combination of implantation of negatively charged fluorine ions and etching of the AlGaN barrier, but this method also suffers from threshold voltage uniformity and repeatability problems. Although method-3 avoids the problem of etching the AlGaN below the gate, growing high-quality and uniform p-type material in the group-III nitride system is extremely difficult, and p-GaN/AlGaN interface trap-related dispersion provides another drawback to this approach.
In addition, GaN devices have been shown to be promising for high frequency applications. Since unpassivated GaN HEMTs have been shown to be dispersive under high speed switching, it is important to engineer these devices to suppress dispersion and therefore optimize the device performance at microwave frequencies.
Moreover, it is important to have GaN devices with structures that reduce gate leakage and therefore increase the breakdown voltage of the device. The reduction in gate leakage will also increase the reliability of these devices.
What is needed, then, are device structures that do not have these drawbacks and can provide these advantages.
The present invention proposes a device structure in the opposite N-polar or (000-1) direction has several advantages over the devices grown in the Ga-polar direction. For example, the proposed device structure in the opposite N-polar direction provides E-mode devices without using a gate recess etch. Threshold voltage is these devices will be controlled by the epitaxial-structure and will not be affected by the processing steps. Furthermore, since the polarization fields are used to deplete the 2-dimensional electron gas (2DEG), p-type doping is not necessary.